The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods for signal synchronization. Merely by way of example, the invention has been applied to light-emitting-diode (LED) drivers. But it would be recognized that the invention has a much broader range of applicability.
In LED back-lighting applications, such as TV with liquid crystal display (LCD), multiple LED drivers often are used to drive multiple LED strings. However, the LED drivers usually have different switching frequencies from each other, which may result in audio noises and/or screen flickers. Hence, synchronization of the LED drivers often is needed to reduce audio noises and/or screen flickers.
LED drivers with lower switching frequencies usually are synchronized with LED drivers with higher switching frequencies. But this scheme often suffers from issues related to un-defined ramping ranges. For example, slope compensation is often needed to make a control loop stable in a continuous current mode. If multiple LED drivers operate at the same switching frequency, the magnitudes of slope compensation in different LED drivers should be the same in order to provide the same switching duty cycle and achieve system stability and LED current precision. But the accuracy of slope compensation usually depends on semiconductor-process variations and component mismatches. Hence, the slope compensation can vary significantly from integrated-circuit (IC) chip to chip. Even though components trimming can be used to reduce the effect of the semiconductor-process variations and component mismatches, such trimming usually is applicable to only fixed frequency operations with increased costs.
Hence it is highly desirable to improve the techniques of synchronizing LED drivers.